My brain is going nuts this late at night...I need to get to bed...lol. I like the rig. I like the release. I like the trucks being identical. I like swapping wheelsets to eliminate possible false readings. BUT If the metal wheelsets do in fact add weight...as was tested by Rick Brodzinsky earlier. And you have them attached to each other. With the truck with metal wheels on top of the truck with plastic wheels...The forces on the bottom truck would be different then when you flip it over and the plastic wheels are on top. I hope that made sense. My mind says the trucks should be independent of each other. Equal weights glued on top of each truck would eleminate any discrepancies when releasing them. I am liking your approach to testing metal wheels and plastic wheels and roll resistance.